Wireless communications system refers generally to any telecommunications system which enables a wireless communication between a user terminal and a network. In a mobile communications system the users are able to move within the service area and to use its services. A typical mobile communications system is the Public Land Mobile Network (PLMN). Often the mobile communications network is an access network providing a user with a wireless access to external networks, hosts, or services offered by specific service providers.
In second-generation mobile systems, such as the Global System for Mobile Communication (GSM), speech and data are transmitted in a digital form. In addition to conventional speech transmission, digital mobile communication systems provide a plurality of other services: short messages, facsimile, data transmission, etc.
The general packet radio service GPRS is a new service in the GSM system (Global System for Mobile communication). A subnetwork comprises a number of packet data service nodes SN, which in this application will be referred to as serving GPRS support nodes SGSN. Each SGSN is connected to the GSM mobile communication network (typically to a base station controller BSC or a base station BTS in a base station system) so that the SGSN can provide a packet service for mobile data terminals via several base stations, i.e. cells. The intermediate mobile communication network provides a radio access and packet-switched data transmission between the SGSN and mobile data terminals. Different subnetworks are in turn connected to an external data network, e.g. to a public switched data network PSPDN, via GPRS gateway support nodes GGSN. The GPRS service thus allows to provide packet data transmission between mobile data terminals and external data networks when the GSM network functions as a radio access network RAN.
Third-generation mobile systems, such as the Universal Mobile Communications system (UMTS) and the Future Public Land Mobile Telecommunications system (FPLMTS), later renamed as IMT-2000 (International Mobile Telecommunication 2000), are being developed. In the UMTS architecture an UMTS terrestrial radio access network, UTRAN, provides User Equipments UE with a wireless access to one or more core networks (CN). The UTRAN consists of at least one radio network controller RNC and a multiplicity of base stations BS. The core networks using the UMTS radio access network include the GSM and the GPRS.
FIG. 1 is a block diagram which gives an overview of the user equipment registration and connection principles within the UMTS in a circuit switched (CS) service domain (e.g. a GSM core network) and a packet switched (PS) service domain (e.g. a GPRS core network). As in conventional GSM/GPRS systems, user identification, authentication and key agreement will take place independently in each service domain. In the CS service domain a circuit switched connection is set up between the UE and a third-generation (3G) mobile services switching center (MSC) or an integrated MSC/VLR (Visitor Location Register). In the PS domain a packet switched virtual connection is established between the UE and a serving GPRS support node (SGSN). In FIG. 1, the CS and PS service domains have a common subscription database HLR (Home Location Register) which stores subscriber data for the users of the UEs. A part of the subscriber data is copied to the CS and PS service domains (e.g. to the VLR and SGSN, respectively) when the UE registers to the corresponding service domain. The security aspects of the UMTS are defined in the technical specification 3G TS 33.102 version 3.4.0 by the 3rd Generation Partnership Project. This document is incorporated by reference herein. Authentication and key agreement are illustrated in chapters 6.3 and 6.4. of the 3G TS 33.102 document. User traffic can be ciphered using the cipher key agreed for the corresponding service domain as illustrated in chapters 6.6. and 6.7 of the 3G TS 33.102 document.
In the UMTS, also unciphered calls or sessions are possible if agreed by the UE and the network. In wireless systems eavesdropping at the air interface is a real problem. This is especially the case in packet data transmission systems like the GPRS where authentication is carried out at the beginning of the virtual connection. A virtual connection means that the UE and the SGSN have created packet data protocol (PDP) contexts which enable to transmit data packets to and from the UE at any time, while the physical channel is reserved only for the actual transmission, which is called a session herein. No authentication is carried out for individual sessions. This approach allows a transmission without delay and optimizes the use of the radio resources at the air interface. The drawback in this approach is that a third party terminal or base station can in principle transmit or receive data for eavesdropping purposes. Also in circuit switched calls, the user may wish to use ciphering in all situations.
Therefore, there is a need for a reliable method for controlling unciphered calls or sessions.